Stressed skin structural diaphragm

ABSTRACT

A roof or ceiling structure for buildings in the nature of a &#34;space frame&#34; or three-dimensional truss, including substantially planar panels and supporting struts and a method for producing the structure on-site. The panels serve a dual purpose as structural members interconnecting the struts and as surface elements for a roof or ceiling. Two panels spaced parallel to one another are interconnected by equiangular struts to form an array of tetrahedrons throughout the three-dimensional structural member. Each panel is made up from sheet material, and the seams between panels are accommodated by rotating each layer of panels relative to those adjacent to it so that the seams overlap abutting sheet surfaces in differing angular attitudes.

BACKGROUND OF THE INVENTION

This invention relates to "space frames", used as self-supporting roofs or ceiling systems in buildings, where an unobstructed span is desired in both length and width. A space frame includes upper and/or lower panels interconnected by struts. The substantially plane panels or "skins" are loaded in compression or tension, depending upon the three-dimensional truss loading pattern of a particular system.

This disclosure is concerned primarily with the production of such a system from plywood panels, which typically are supplied in rectangular sheets of limited width and length. One task faced by any designer of a plywood structure is that of utilizing the limited dimensions of available plywood in the production of spans that are a multiple of such limits. While plywood can be produced at the mill in extended length sheets by use of scarf joints and other end-to-end bonding techniques, these methods require expensive presses and forming machinery. They are only of limited value where increased dimension is necessary in both length and width of a sheet. Such methods normally cannot be carried out at a building site, and the ultimate sheet size that can be produced at a mill is limited by size limitations during shipment from the mill to a building site.

Space frames in general have been previously utilized to span a building area. Many of these have used self-supporting skeletal frames covered by a non-structural skin of lightweight metal or other sheet material. The key feature herein is the novel utilization of conventional sheets of plywood or other structural sheet material to produce large areas of panels for inclusion in such systems.

SUMMARY OF THE INVENTION

The disclosed product or system basically comprises two or more channels formed from rows of structural sheet material, the panels being overlayed on one another and spanning the desired length and width of the building area. Each layer of material has equally spaced points located along its full length and width. These points are located along lines developed parallel to one edge of each row and offset longitudinally midway between the points along each line adjacent to it. The spacing between the lines and points is such that any three adjacent points form an equilateral triangle. The layers are superimposed on one another with these points of each layer coincident to one another and with the respective side edges of the rows of sheet material in abutting layers being angularly offset from one another by 120°. Thus, the edges of the sheets angularly cross one another and no two sheet edges in any layer can be in a coincident position. When the layers are bonded to one another, the overlapping edges assure surface-to-surface force transmission throughout the area of the resulting panel.

The above panel is utilized as the skin of a structural diaphragm. This is accomplished by connecting tetrahedral struts to the points located on the panel and by interconnecting the outer ends of each strut to complete a three-dimensional truss configuration. When desired, this latter interconnection can be accomplished by use of a second panel identical to and parallel to the first panel.

The method of producing the structural diaphragm comprises first the step of locating connection points or apertures in each row of sheet material. In the case of plywood sheets, the points are located parallel to one long edge of each sheet and are spaced transversely the height of a single sheet. They are spaced equally from one another as points of equilateral triangles. The rows of sheet material are then placed in abutment with one another, with the connecting points of one row being longitudinally offset midway between those of the rows adjacent to it. The connecting points along adjacent rows of sheets form equilateral triangles. Multiple layers are produced in this manner and superimposed one on the other with the side edges of the rows angularly offset 120° from one another and the connecting points coincident. These points can then be secured to tetrahedral struts and to a second spaced panel to complete a structural system for a roof or ceiling or both.

A first object of this invention is to provide a practical method of utilizing plywood or similar sheet material conventionally produced in sheets of limited length and width, wherein the material can be incorporated into large panel areas involving substantial multiples of such limited dimensions.

Another object of this invention is to provide practical hardware for fabricating stressed skin structural diaphragms from plywood.

Another object of this invention is to provide a system whereby the components of the structural diaphragm can be fabricated and assembled at the building site.

These and further objects will be evident from the following disclosure, taken together with the accompanying drawings. The drawings illustrate the conceptual basis of the disclosure and several variants in hardware and fabrication.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary plan view of a single layer of sheets for a diaphragm;

FIG. 2 is a plan view showing the superimposed layers in a single panel;

FIG. 3 is a fragmentary perspective view showing a panel and upstanding strut assemblies;

FIG. 4 is a similar perspective view, showing the completed diaphragm;

FIGS. 5-9 are simplified schematic views showing various area formations;

FIG. 10 is an additional schematic view showing a plan having an open center;

FIG. 11 is a schematic view illustrating preparation of a row of sheets;

FIG. 12 is a perspective view of a bracket for the panels;

FIG. 13 is a plan view of the bracket shown in FIG. 12;

FIG. 14 is a side view of the bracket shown in FIG. 12;

FIG. 15 is a sectional view through a panel at the strut connection point;

FIG. 16 is a fragmentary plan view showing attachment of the bracket to a panel;

FIG. 17 is a sectional view through a connecting point in a lower panel assembly, showing a modified bracket assembly;

FIG. 18 is a view similar to FIG. 17, showing a modified bracket assembly for an upper panel; and

FIG. 19 is an exploded perspective view of still another embodiment of a bracket for the panel.

DESCRIPTION OF THE PREFERRED EMBODIMENT

This invention relates to a stressed skin structural diaphragm or "space frame", wherein the structural skin is produced from repeated sheets of material placed end to end in rows parallel to one another. The sheets abut one another in an edge-to-edge relationship across the full width of the building area to be spanned thereby.

One key feature of this system is the utilization of three-dimensional tetrahedral structures interlocking parallel spaced panels, with the panels themselves serving as planar struts or connectors in the tetrahedral system.

In order to produce a large area panel from conventional sheets of plywood or other structural materials, such as rectangular sheets of plywood, wood veneer, particleboard or other wood-based material, each panel is constructed in a multiple number of layers. This number is preferably three, but can be as few as two where adequate surface-to-surface bonding between the layers can be assured, or can be any greater number of layers as required for strength purposes. Three layers produces a balanced structural system in connection with the tetrahedral supports.

In general, this method of producing a stressed skin structural diaphragm comprises the following steps: (1) arranging a plurality of longitudinal rows of sheet material in side-by-side abutting positions; (2) locating equally-spaced points along at least one line parallel to one corresponding side edge of each row of sheet material, the points along each adjacent pair of rows being offset longitudinally midway between one another, whereby the points located along adjacent rows form equilateral triangles; (3) arranging the rows of sheet material in parallel side-by-side abutting positions covering the area to be spanned; and (4) arranging a second layer of identical rows of sheet materials over the first, with the points located in the second layer coincident to the points located in the first layer and with the corresponding side edges of the two layers being angularly offset relative to one another.

FIG. 1 illustrates a first layer of a panel. It is comprised of rectangular sheets of material 10 arranged end to end in longitudinal rows indicated as 10a, 10b, 10c, etc. These rows are arranged parallel to one another with their adjacent longitudinal edges in abutment. The transverse edges of each sheet abut those sheets adjacent to it in each row. The longitudinal edges in FIG. 1 are designated by the numeral 11 and the transverse edges by the numeral 12. In the United States, plywood sheets are conventionally produced with a longitudinal dimension or length of eight feet and a transverse dimension or width of four feet. The object of this disclosure is to describe a system for producing large panels having both length and width dimensions which are many times these conventional sheet dimensions, the ultimate dimensional limits of the panel being determined by the design load limits of the resulting three-dimensional truss.

Located along each row of material are connecting points. The connecting points are schematically illustrated in FIG. 1 by circles 13. They are equally spaced along imaginary or real lines 14 parallel to the longitudinal edges 11 of that row. The connecting points 13 along adjacent lines 14 are staggered longitudinally so as to be located or interspersed midway between one another. Every point 13 across the entire panel is equally spaced from every other point adjacent to it. Any two points 13 on one line and their intermediate points 13 on adjacent lines form equilateral triangles. The height of each triangle is the lateral or transverse spacing between the adjacent lines 14. The spacing of adjacent points 13 along line 14 equals the straight line space between each point 13 and the intermediate points 13 on an adjacent line 14.

In the illustrated forms of this invention, the spacing between lines 14 equals the transverse width of each sheet 10. In this manner, each line 14 is spaced inwardly of the adjacent longitudinal sheet edges 11 by an equal distance. This facilitates construction of the panel, since the points are located identically along each row of sheets. However, the spacing illustrated constitutes the maximum spacing that is practical between lines 14. Lines 14 can be spaced apart by a distance less than the sheet width, which would then require re-scaling of the separation between points 13 along the respective lines 14. A lesser spacing between lines 14 might also pose fabrication difficulty in locating lines 14, which would vary in position along each row. Were the spacing between line 14 to be greater than the width of the sheets, certain rows of sheets would eventually not contain any lines 14 or points 13. It is intended that every row of sheets contains at least one line of connecting points 13.

To produce the panel utilized in this system, it is preferable that three separate layers be fabricated as shown in FIG. 1 and be superimposed one on the other in surface-to-surface contact as shown in FIG. 2. As seen in FIG. 2, the points 13 on each layer are coincident or coaxial along lines perpendicular to the surfaces of the layers. In FIG. 2, which is a plan view looking on the panel section, the sheets 10 in the uppermost layer are shown in full lines, the sheets in the intermediate layer are illustrated by alternate long and short broken lines, and the bottom layer sheet boundaries are illustrated by short broken lines. While the points 13 are coincident, the edges of the sheets in adjacent layers are rotated or turned 120° with respect to one another. This assures substantial surface overlapping between the adjacent layers and angular offsetting of the seams between the abutting sheets in the layers.

FIG. 3 illustrates the next basic step in production of the structural diaphragm. This is the addition of outstanding equiangular struts 15, each of which has an effective connected length between its ends identical to that of every other strut. This effective length also is identical to the spacing between the points 13 located on the panel layers. The struts are arranged along one side of the panel and project outwardly therefrom. They are connected in clusters or groups of three each at a connecting point coincident to one of the points 13. They diverge equiangularly from the point 13 at an angle of 60° with respect to the plane of the panel surface. Their outer ends are joined also in groups of 3 and the struts 15 form tetrahedral structural elements across the entire area of the panel. Struts 15 are rigid. They can be constructed from metal, plastic, wood or any other suitable material. The base of each tetrahedron is also rigid, particularly in view of the fact that every pair of connecting points in the panel includes at least one layer of unbroken sheet material spanning the two points.

As illustrated in FIG. 4, the basic features of the structural diaphragm are completed by a second panel identical to that illustrated in FIG. 2. In FIG. 4, the roof-ceiling system includes an upper panel 16 and a lower panel 17. Any three struts and included panel area constitutes a tetrahedron, having even stress distribution in both the longitudinal and transverse directions. The upper panel will normally be in compression and the lower panel in tension, these forces being distributed throughout the struts 15 in the well-known force arrangements of a three-dimensional truss.

It is not absolutely essential that the effective length of strut 15 be identical to the spacing between the points 13. The effective length of struts 15 can be longer or shorter than the point spacing, however this will vary the force relationships between the struts and panels. Furthermore, the angles formed between the points 13 need not necessarily be equilateral triangles. The spacing between lines 14 might be greater or less than the spacing between points 13 along line 14. However, this will introduce directional strength properties to the resulting system and will vary the load distribution in either the transverse or lateral direction.

The above system lends itself exceptionally well to the production of open span buildings of one or more stories having an area in the shape of a square, rectangle, triangle, or six-sided building. Other geometric shapes and combinations of such shapes are also possible by interconnecting the tetrahedrons across the desired area. Examples of some spatial outlines that can be constructed using this general disclosure are shown in FIGS. 5-9, which illustrate only the outline of each panel and the longitudinal edges of the rows of sheets in the three layers. FIG. 10 shows a more complex arrangement of a span including an open central area. Interior supports are unnecessary in any of these arrangements, because the rigid spatial frame itself is self-supporting. Where it is not essential to provide both a roof and ceiling surface, one of the panels can be eliminated and rigid struts or other structural members in a plane parallel to and spaced from one panel can be substituted. Thus, where a roof is desired without a ceiling, the upper panel can be constructed as shown in FIG. 2 and the lower connected ends of the struts 15 can be connected to one another by rigid horizontal struts in a plane triangular pattern.

While not limited to plywood structures, this system has been designed specifically for utilization of plywood sheets. Sheets of wood veneer, particleboard, or other wood-based material can be substituted in place of plywood. Furthermore, composite sheets of metal or plastic resins or any plane sheet material supplied in repetitive sheets or rows can be utilized in this structure.

FIG. 11 schematically illustrates the steps involved in preparation of each row of sheets 10. The sheets are fed from a stack 20 in end-to-end abutting relationship with one another along a row moving longitudinally to the right in FIG. 11. They are drilled sequentially by a drill press 21 or other suitable equipment capable of producing a line of identically-spaced apertures or holes 22 along the row of sheets. With plywood sheets 4 feet by 8 feet in dimension, an equilateral triangle having a height of 4 feet has an aperture spacing (center to center) of 4.618836 feet or 4 feet, 7.426032 inches. The spacing between the apertures is indexed by a detector 23, which detects the presence of an aperture in a sheet and automatically controls operation of drill 21.

A bracket 24 is placed on the first layer or course of sheets 10. The bracket is shown in detail in FIGS. 12-14. In its basic form, the bracket 24 is bent from a single sheet of metal. It is folded in an "accordion-style", forming a series of slotted shelves with back surfaces 120° offset from one another. The formed bracket 24 includes four parallel horizontal bearing plates 25 which are triangular in shape and which are integrally joined by three perpendicular vertical shear plates 26. Adjacent shear plates 26 form angles of 120° relative to one another. Each triangular bearing plate 25 is open along two sides and closed along one side by a shear plate 26.

The brackets 24 are placed on the first layer of sheets as shown in FIG. 11. The lowermost shear plate 26 abuts the longitudinal edge of the sheets 10. Two bearing plates 25 overlap the upper and lower sheet surfaces and have apertures 27 in registry with the individual apertures 22. The layer of sheets is then formed by positioning a plurality of rows of sheets adjacent to one another in transverse abutment as shown in FIG. 1, with the brackets 24 of adjacent rows being centered midway between one another.

Each row of sheet material therefore has a plurality of equally-spaced points located thereon along the full length of the row, the points being located along one or more lines on each row of sheet material parallel to the side edges of the row, the points along each line being offset longitudinally midway between the points along each line adjacent to it and the spacing between the lines and points being such that any three adjacent points form an equilateral triangle.

Each layer of the panel comprises a plurality of longitudinal rows of sheet material having parallel side edges extending longitudinally along that area, said rows of sheet material being arranged in co-planar, side-by-side abutting positions with the rows of sheet material adjacent thereto.

The sheets of plywood for the subsequent layers are also drilled in the manner described with respect to FIG. 11. They are then fitted into the brackets 24, utilizing the intermediate slot of each bracket 24 for guidance of the intermediate layer of plywood and the upper slot for guidance of the upper layer. Each layer is aligned with its longitudinal side edges abutting a row of shear plates 26 and with its apertures 22 coincident or coaxial with the apertures of the remaining layers. Each layer is therefore superimposed on an adjacent layer with the points located thereon at apertures 22 coincident with one another and the respective side edges of the rows of sheet material being angularly offset 120° from one another about said points. Thus, when the three layers are in position within the brackets 24, connecting bolts 28 can be inserted through the sheets and brackets to attach the joined struts 15 (FIG. 15).

The strength of each structural diaphragm is dependent upon surface bonding of the layers of plywood. This can be accomplished by the application of glue or adhesive during the formation of each row of sheets 10. FIG. 11 schematically shows a glue applicator at 30, used to apply suitable adhesive to the upper surfaces of first and second layers of plywood. The adhesive may be thermosetting or thermoplastic, or might be of a type that sets simply by drying. Besides adhesive, nails or staples can be used to assure initial contact of the layers of plywood. Portable radio frequency heating devices can be used in the field where the adhesive requires heat for curing. Bonding of sheets of plywood is well known and many available techniques can be adaped to production of this system.

FIG. 16 illustrates one method of providing additional tensile connection between brackets 24. It simply involves the use of a metal strap 31 wrapped about each shear plate 26 and spot-welded or otherwise connected to it. The straps extend between adjacent brackets 24 and similar straps 31 are attached to each shear plate 26 in the various layers of the system. The straps 31 reinforce the ability of the plywood to withstand the tensile forces exerted through the brackets 24 in the structural system.

FIGS. 17 and 18 illustrate specialized arrangements for fabricating the connecting points on site. They are concerned with the erection of the system on a slab. Where the system is built directly on a slab and lifted into place, it is necessary that the bottom skin be fabricated in such a manner that the connecting struts and layers can be properly located without access to the underside of the structure. In addition, the top skin must be fabricated with a minimum of access to the underside thereof.

Referring to FIG. 17, the bearing plates 32 are spaced apart a distance equal to the thickness of each layer of plywood 33. They are connected by shear plates 34 offset angularly from one another by 120° as discussed above. The bearing plates 32 and shear plates 34 can be formed integrally from light metal stock, or can be separable elements (FIG. 19).

The lowermost bearing plate 32 is located with respect to a cylindrical aperture cut through the bottom layer of plywood 33 at each connecting point. Plate 32 has a nut 35 fixed to it, either by spot-welding or other suitable means. The nut 35 includes an inner threaded recess for ultimate reception of a connecting bolt.

To begin fabrication of the lower skin, the apertured sheets of plywood are laid over the supporting ground surface in abutting side by side positions, with the brackets in place. The bearing plates 32 and shear plates 34 are initially expanded, and bent into the required parallel relationship as the various layers are put into place. The lowermost layer receives a spacer 36 within each aperture of the plywood sheets. The spacer 36 is annular and has a central opening complementary to the surfaces of nuts 35. It locates the lowermost layer of plywood about the central nuts 35.

The second layer of plywood 33 is then placed over the first, the edges of the plywood being offset 120° from one another. The plywood is properly located by a larger annular spacer 37 that fits over a projection on each nut 35. This arrangement requires a larger aperture in each sheet of plywood in the central or second layer of each skin. Finally, the uppermost layer of plywood 33 is fitted about spacers 38, again located by projections at the top of spacers 37. In this manner, the three layers of plywood are oriented along the shear plates 34 and their connecting points are coaxial or coincident with one another along the vertical axis of each nut 35. The layers are then in readiness for reception of a connecting bolt 40 which is received through apertures at the lowermost ends of three struts 41. A heavy enlarged washer 42 is interposed between the lower ends of the struts 41 and the uppermost bearing plate 32.

After the struts 41 have been mounted in place, their upper ends are temporarily connected by means of an inverted anchor cup 43. The center of each cup 43 is coincident with the connecting points in the upper skin. Each strut 41 is connected to the downwardly open lip of cup 43 by a bolt 44. The struts 41 have upper apertures aligned along the axis of cup 43 for ultimate reception of a connecting bolt. The bolts 44 are not designed to transmit the system load from struts 41, but are intended to serve as temporary support during development of the upper skin.

As shown in FIG. 18, the upper skin is fabricated along the co-planar upper surfaces of the cups 43. The plywood layers 45 are received between bearing surfaces 46, with the plywood side edges abutting shear plates 47. Interfitting spacers 48, 50 and 51 are located within the respective apertures of the plywood layers as the skin is built-up in place. Finally, a bolt 52 is inserted through the struts 41 and the stack of spacers, and is threadably engaged by a nut 53. Insertion of bolts 52 requires that one crawl between the two skins of the system, but sufficient space will normally be available for this operation. Nuts 53 can be either attached to the uppermost bearing plate 46 or can be held stationary by a spanner wrench during tightening of bolts 52.

FIG. 19 shows an optional method of fabricating the bearing plates and shear plates as separate elements for each layer. The intermediate bearing plates 54 have projections 55 and 56 extending oppositely along two sides. They interfit similar projections on adjacent bearing plates to complete an assembly that is the equivalent of the integral bearing plates and shear plates described above. The top and bottom bearing plates 57 have only one set of projections 58 which interfit the adjacent projections to produce a shear plate assembly. This arrangement allows the bearing plates to be assembled on the sheets of plywood as the sheets are being laid up in place during production of the respective skins.

To accomplish a large span arched structure, preassembled units could be built in hexagonal shapes with a built in camber, allowing for easy ground assembly, and can then be hoisted into position in modules. Camber in the individual module is achieved by increasing the spacing between the long edge of the 4 feet by 8 feet plywood on the first course of the top chord. These sheets would run perpendicular to the direction of the camber. This increased distance would create an isosceles triangle instead of an equilateral triangle. The actual distance between holes on the other two sides of the isosceles triangle would be dictated by the radius of the arch. This then would increase the distance between the holes on the 2nd and 3rd courses of the top chord. It would also change the angle of the shear plates.

While considerable detail is illustrated with respect to the connecting points, it is to be understood that many different mechanical devices might be used to connect the struts and skins to one another in the present system. The physical details shown in these drawings are intended to serve only by way of example, and are not to limit the scope of the invention, which is set out in the claims that follow. 

Having thus described my invention, I claim:
 1. A structural diaphragm, comprising:a plurality of layers of structural sheet material overlaid on one another across the length and width of an area to be spanned; each layer comprising a plurality of longitudinal rows of sheet material having parallel side edges extending longitudinally along that area; each of said rows of sheet material being arranged in a co-planar, side-by-side abutting position with respect to the rows of sheet material adjacent to it; each row of sheet material having a plurality of equally-spaced apertures formed through them along the full length of the row, the apertures being located along at least one line on each row of sheet material, each line being parallel to the side edges of the row containing it, the apertures along each line being offset longitudinally midway between the apertures along each line adjacent to it and the spacing between the lines and apertures being such that any three adjacent apertures form an equilateral triangle; each layer being superimposed on an adjacent layer with the apertures located thereon coincident with one another and the respective side edges of the rows of sheet material being angularly offset 120° from one another about said apertues.
 2. A structural diaphragm as set out in claim 1 wherein the apertures formed through each row of sheet material are located along a single line spaced inwardly an identical distance from one corresponding side edge of the row.
 3. A structural diaphragm as set out in claim 1 including three layers superimposed one on the other and co-extensive in length, width and peripheral configuration.
 4. A structural diaphragm as set out in claim 1 wherein each row is comprised of rectangular sheets arranged in end-to-end abutting relationship with one another.
 5. A structural diaphragm as set out in claim 1, further comprising:a plurality of rigid struts each having an effective length between the ends thereof equal to the separation between adjacent apertures along said lines; each strut being connected in a group of three struts joined to one another, each group being joined to the layers of sheet material through a selected one of said apertures and extending outward to one side of the layers of sheet material, said struts in each group diverging equiangularly relative to one another to form tetrahedral structures in combination with the layers of sheet material; and means joining the remaining ends of the struts along a plane parallel to and spaced from the layers of sheet material.
 6. A structural diaphragm as set out in claim 5 wherein said last-named means comprises a second panel identical to that set out in claim 1, the remaining ends of said struts being connected to said second panel in groups of three struts at each point located on the second panel.
 7. A structural diaphragm, comprising:a plurality of layers of structural sheet material; each layer including a plurality of planar sheets arranged along parallel rows in edge-to-edge abutting positions fully covering the desired panel area; each row of sheets having apertures formed through them along lines parallel to one corresponding edge of the row, the apertures being located on said sheets such that any three adjacent apertures along any two adjacent lines across the layer form the corners of an equilateral triangle; the respective layers being overlaid one on the other, with said rows of sheets along adjacent layers being turned 120° with respect to one another about each aperture, the apertures of the layers being in coaxial registry with one another.
 8. A structural diaphragm as set out in claim 7, wherein the sheets are rectangular sheets of wood-based material.
 9. A structural diaphragm as set out in claim 7 wherein the height of the equilateral triangles formed by said apertures equals the width of a single row of sheets. 